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The objectives of depressurizing (blowing down) the process equipment in the event of a fire are to:

Reduce the combustible inventory; reduce the time that a jet fire from a leak can be sustained; avoid a vessel rupture caused by the fire heating up the vessel’s metal wall and thereby lowering the metal’s yield stress below that induced by the vessel’s internal pressure.

The objectives of depressurizing (blowing down) the process equipment in the event of a fire are to:

-Reduce the combustible inventory

-Reduce the time that a jet fire from a leak can be sustained

-Avoid a vessel rupture caused by the fire heating up the vessel’s metal wall and thereby lowering the metal’s yield stress below that induced by the vessel’s internal pressure.

Note that relief valves will only protect a vessel from rupture during a fire as long as the metal wall is maintained within its design temperature range by the boiling of its bubble-point liquid inventory. If the flame impinges on the wall above the liquid line (or if the vessel contains a non-volatile liquid) the metal wall will be heated up and the metal yield stress will be reduced accordingly. When the yield stress has been reduced below that induced by the internal pressure, then despite the internal pressure being maintained at its design value, the vessel will rupture.
The blowdown requirement is specified by API RP 521 3rd edition [51] is a reduction to 50% of design pressure in 15 minutes. Some plant Operators however prefer to retain the first edition requirement of a reduction to 100 psig within 15 minutes because of concerns over the high heat fluxes inflicted by jet fires within the confined spaces of offshore platforms. In either case, the blowdown continues after the 15 minutes has been reached so that eventually the vessel is completely depressurised.

Source: http://www.optimal-systems.demon.co.uk/controlled-blowdown.htm

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